[Heathkit] Re: parasitic suppression - Better Title

[jeremy-ca]

Again and again you are confused and mixing up what is going on. The rod is 
for straight neutralization and is used in many circuits thru 2M The rod is 
bent to or away from the tube as needed; it is simply one plate of a 
feedback capacitor and the tube anode is the other. It is used when only a 
very small capacitance is required. The variable capacitor performs the same 
function when more capacitance is required. NEITHER has anything to do with 
parasitics....PERIOD.

LF parasitics are usually caused when the grid bias choke and its associated 
bypass capacitor create a resonance that is able to pass thru the anode 
circuit with little attenuation. This was very common in amplifier stages of 
the 30's and 40's where bypass capacitors were highly inductive. For this 
reason many designers did away with the choke and replaced it with a carbon 
resistor of 1K Ohms or thereabouts. A ferrite bead over the grid lead will 
also help suppress that oscillation. Ive used a combination of ferrite and 
low inductance ceramic capacitors to tame notoriously unstable RF and IF 
amps in late 20's to mid 30's receivers. An amplifier at any power level, 
from milliwatts to serious QRO can be unstable; the higher the gain the more 
likely it becomes.

Before responding with further confused statements I suggest that you read 
up on some 30-50's application notes from RCA, GE, Philco and others that 
address LF oscillation, it really should not be called a parasitic since it 
is not created inside the tube or the anode tank circuit.

Carl
KM1H



From: "rbethman" <rbethman at comcast.net>
To: <heathkit at mailman.qth.net>
Sent: Sunday, August 26, 2007 3:51 PM
Subject: Re: parasitic suppression - Better Title


> Carl,
>
> I submit that the 6146 *indeed* has required *both* low frequency *and* 
> VHF parasitic suppression.  It is also a 6.3V filament tube that did *not* 
> ever make it into automobiles without a HAM installing a radio for his/her 
> use.
>
> The 6146 *and* the sweep tubes used in Heath equipment, I.E., the HW-12, 
> HW-16 as examples, have in their designs the cure for *BOTH* low frequency 
> and VHF frequency parasitic resonance.  These are NOT some off the wall 
> antique tubes, nor are they NON Heath radios.
>
> In some instances, the neutralization was achieved by a simple aluminum 
> rod in the RF PA enclosure bent as needed to resolve the problem, (DX-60 
> and TX-1 Apache).  In other instances the problem was fixed by a 
> capacitor, air variable, from plate back to grid.  (HW-101 - SB-102).
>
> You and I have been around for some time.  We both know a fair bit of 
> information.  Sometimes we overlook are forget things.  It does NOT take 
> only a Triode, nor 250KW radios to deal with parasitic resonance.  It can 
> even be a TPS-1D radar system!
>
> Bob - N0DGN
>> The low frequency parasitic in vacuum tube ham equipment went out with 6V 
>> electrical systems in cars. Handbooks like to repeat themselves for 
>> several generations before some one catches on and edits. Some of those 
>> antique tubes even took off at audio.
>>
>> And yes, some SS stages can have LF parasitics.
>>
>> Since this IS a Heathkit reflector I am sticking to the tubes they use; 
>> they do not lend themselves to LF parasitics.
>>
>> If you wish to discuss how to tame a 250KW SWBC amp that is a different 
>> story.
>>
>> Carl
>> KM1H
>
>